In a toroidal type continuously variable transmission for a vehicle, such as the one disclosed in U.S. Pat. No. 5,542,890, the speed change ratio is changed by varying the gyration angle of a plurality of power rollers which are squeezed between an input disk and an output disk. Each of these power rollers is supported by a trunnion.
The gyration angles of the power rollers are changed by applying power to the trunnions in a specific direction which lie at right angles to the rotational axes of both the power rollers and the disks. These changes are brought by the loads exerted by the input disk and the output disk upon the power rollers when they are displaced in the aforesaid specific direction.
This mechanism is disclosed in, for instance, S.A.E. Technical Paper No. 901761.
A trunnion drive mechanism is disclosed, for example, in Tokkai Hei 2-283949 published by the Japanese Patent Office in 1990. The continuously variable transmission disclosed in this prior art is a so called double cavity type continuously variable transmission in which two toroidal speed change units are arranged in parallel, and each of these units comprises one set of power rollers arranged symmetrically. Each power roller is supported by a corresponding trunnion, and a piston is formed integrally with each trunnion in order to drive the trunnion in its abovementioned specific direction; hydraulic pressures act via hydraulic conduits upon these pistons as shown in FIG. 23.
In this figure, the pistons FR and FL are linked to trunnions which support the power rollers of one of the speed change units. In the same manner, the pistons RR and RL are linked to trunnions which support the power rollers of the other one of the speed change units.
The hydraulic pressures for driving the pistons are supplied from a speed change control valve S/V as an upshift pressure Pu or a downshift pressure Pd.
As will be understood from this figure, the hydraulic pressures always act upon the pistons FR and FL in opposite directions. In the same way, the hydraulic pressures always act upon the pistons RR and RL in opposite directions. Due to this, the two trunnions in the same speed change unit always shift in mutually opposite directions as shown by solid arrows in the figure.
Now, if due to an external disturbance a force F should act upon the piston RR in the direction shown in the figure by a dashed arrow, a pressure difference is set up between the hydraulic chambers on opposite sides of this piston RR, and this pressure difference is transmitted to the other pistons via the hydraulic pressure conduits, so that forces act upon the other pistons as shown by dashed arrows in the figure.
If the solid arrows in the figure are taken as indicating the upshift direction, then, although the external disturbing input force upon the piston RR is in the upshift direction, the forces which act as a result upon the other pistons FR, FL, and RL are in the downshift direction, as shown by the dashed arrows in the figure.
Accordingly, this type of external disturbing input force spoils the symmetrical displacement of the trunnions, and is a principal cause leading to vibration of the power rollers.